An Id protein was found as a myogenic regulatory factor suppressing the activity of a muscle-specific gene in mouse C3H10T1/2 fibroblasts [Cell, Vol. 61, 49-59 (1991)]. A subsequent research revealed that Id proteins are recognized in many kinds of cells during proliferation and decrease upon removal of sera and growth factors, and differentiation of cells [Proc. Natl. Acad. Sci. USA. Vol. 88, 1815-1819 (1991)].
From the above-described facts, the presence of an Id protein in cells and the expression of an Id gene that codes for the Id protein can be positioned as an index for determining the state of cell proliferation, and the state of cell differentiation and aging.
Three kinds of genes, Id-1, Id-2 and HLH462 have presently been known as mouse Id genes. However, only a human Id-2 gene corresponding to the mouse Id-2 gene has heretofore been known as an Id gene present in human cells [Proc. Natl. Acad. Sci. USA, Vol. 89, 1512-1516 (1992)]. Therefore, several unknown human Id genes have been considered to exist. In view of further applications of Id genes and knowledge about the Id genes to fundamental experiments on the human and human cells, diagnoses and therapies, a problem to be early overcome has been to acquire unknown human Id genes.